Polymer electrolyte fuel cells are expected as a future energy generating device because they have high energy conversion efficiency, are clean, and are quiet. Further, because the polymer electrolyte fuel cell has high energy density and the low operating temperature, in recent years, not only application to automobiles, household power generators, or the like, but also application to portable electrical apparatuses such as mobile phones, notebook personal computers, or digital cameras are taken in consideration. The polymer electrolyte fuel cell is capable of driving the portable apparatuses for a long time compared to a related-art secondary battery, thereby receiving attention.
Although the polymer electrolyte fuel cell has an advantage of being able to operate at an operating temperature of 100° C. or less, there is a problem in that, with a passage of power generation time, a voltage is gradually reduced, and the power generation stops at last.
Such a problem results from a so-called “flooding phenomenon” in which water generated by reaction remains in a void of a catalyst layer, and the water fills the void of the catalyst layer to block supply of a fuel gas serving as a reactive substance, thereby reducing a voltage to stop power generation reaction in the end. Flooding is apt to occur especially in a catalyst layer of a cathode side in which water is generated.
To use the polymer electrode fuel cell as a compact fuel cell for an electrical device for a practical purpose, the entire system has to be made compact. Especially, when a fuel cell is mounted on a compact electrical device, not only the entire system but also the cell itself have to be miniaturized. Accordingly, a method (air breathing) of supplying air through vent holes to a cathode by natural diffusion without using any pump or blower is considered promising.
When this method is used, because generated water is discharged out of the fuel cell only by natural evaporation, the generated water frequently remains in the catalyst layer to cause flooding.
Thus, improvement of gas diffusion of the catalyst layer, especially generated water scattering thereof, to suppress flooding is an important factor which determines performance stability of the fuel cell.
In this regard, Japanese Patent Application Laid-Open Nos. 2004-327358 and 2004-039474 disclose technologies involving improving gas diffusion and generated water scattering by disposing a water discharge groove in a catalyst layer.
International Publication No. WO 2006004023 discloses a method of forming a porous catalyst layer having a dendritic shape by using sputtering or ion plating. International Publication No. WO 2006004023 discloses a technology involving improving gas diffusion and generated water scattering by setting a thickness of the catalyst layer to several μm, which is smaller than that of a conventional platinum carrying carbon catalyst (several ten μm), and shortening a substance diffusion path of gas and water droplets.
According to M. S. Saha A. F. Gulla, R. J. Allen and S. Mukerjee, Electrochim. Acta, 51 (2006) 4680., when a catalyst layer is formed by ion beam assist deposition (IBAD), by using a mask, the catalyst layer is patterned to be divided into a catalyst forming area and a catalyst nonforming area. According to M. S. Saha A. F. Gulla, R. J. Allen and S. Mukerjee, Electrochim. Acta, 51 (2006) 4680., scattering of generated water and an output of the fuel cell are thus improved.
According to an example of International Publication No. WO 2006/004023, by using a transfer sheet made of polytetrafluoroethylene (PTFE) as a substrate, a porous catalyst layer is formed by vapor-phase deposition, and then transferred to a polymer electrolyte membrane to manufacture a membrane electrode assembly (hereinafter, referred to as “MEA”).
However, the inventors of the present invention have zealously conducted a study to find that in the case of the embodiment of International Publication No. WO 2006/004023, a dense layer is formed with a thickness of about 0.1 to 0.2 μm near the PTFE sheet side of the porous catalyst layer. In this description of the present invention, the dense layer means a layered area which is porous but lower in porosity than other areas of the porous catalyst layer.
The inventors of the present invention have found a problem that when a fuel cell unit is assembled, the dense layer inhibits substance diffusion between the porous catalyst layer and a gas diffusion layer.
The inventors of the present invention have also found that when the MEA is manufactured, a swelling action of the electrolyte membrane generates many cracks of 1 μm or more in width in the catalyst layer at random. It is probable that in the porous catalyst layer, gas and water droplets are diffused in an in-plane direction of the catalyst layer, and transferred with the gas diffusion layer through the cracks.
In other words, conventionally, many places in which lengths of substance diffusion paths are larger than the thickness of the catalyst layer have been present, causing a reduction in substance diffusion of the porous catalyst layer.
Even if the manufacturing methods of Japanese Patent Application Laid-Open Nos. 2004-327358 and 2004-039474 are simply combined with the catalyst layer forming method of International Publication No. WO 2006/004023 to dispose a water discharge groove, a dense layer is still formed near the transfer sheet of the catalyst layer to inhibit substance diffusion, thus providing no solution to the problem.
According to the manufacturing method described in Japanese Patent Application Laid-Open No. 2004-039474, the transfer sheet is bent into a mold form by using a mold, thereby forming a nontransfer portion, that is, a groove of a catalyst. Thus, widths and intervals of grooves to be formed are larger twice or more than a thickness of the transfer sheet. According to Japanese Patent Application Laid-Open No. 2004-039474, no strength can be obtained to endure a manufacturing operation unless a thickness of the transfer sheet is 10 μm or more. Thus, this method has a problem in that widths and intervals of grooves have to be larger than 20 μm.
To improve substance diffusion of the catalyst layer described in International Publication No. WO 2006004023, an interval of the grooves has to be set almost equal to the thickness (several ten μm) of the catalyst layer. Thus, even if it is simply combined with the method described in Japanese Patent Application Laid-Open No. 2004-039474, this method provides no solution to the problem.
If a width of the groove is large, i.e., 100 μm, it is not suitable because a capillary force is reduced to cause easy remaining of generated water in the groove. Thus, the method provides no solution to the problem.
Formation of a catalyst nonforming area (this area is functionally a water discharge groove) by using mask deposition as described in M. S. Saha A. F. Gulla, R. J. Allen and S. Mukerjee, Electrochim. Acta, 51 (2006) 4680. is a simple method. However, a catalyst formed on the mask is useless, causing a cost problem. Even if the catalyst formed on the mask is recovered to be used again, recovery costs are considerable. Mask forming expenses also contribute to the cost increase.
The conventional technologies have had the above-mentioned problems. Thus, there has been a demand for a practical technology capable of improving substance diffusion and generated water scattering of the catalyst layer having the dendritic shape as described in International Publication No. WO 2006/004023, and catalyst use efficiency.